Arguments about the finest operas of our time, or the best orchestral works, would be long and indeterminate. But similar discussion of the piano repertory would be pithy and conclusive: György Ligeti's ongoing series of Etudes, 18 of them so far (all played here by Pierre-Laurent Aimard) stands head and shoulders above any other works composed for the instrument in the past 40 years.

Though the pieces, divided into books, of which the third is still in progress, do not need to be played as a totality, they gain stature from such an approach, and the architecture of the cycle is clarified: the haunted melancholy of Automne à Warsaw at the end of the first book is one landmark in the sequence; L'Escalier du Diable, ferocious and torrential, in the second, is another. The pieces so far completed in the third suggest a paring down, a new simplicity.

Technically it is all wonderfully allusive. Ligeti's fascination for music that disappears before the ears is regularly exploited. Several Etudes, like the exquisitely coloured Arc-en-Ciel evaporate in the highest reaches of the instrument, others are swallowed up by the depths of the bass. The nagging cross rhythms of Fém finally expire at both ends of the keyboard simultaneously. So much of the texture and rhythmic variety is generated by canons, packed as tightly together as they were in Ligeti's early orchestral music yet to a totally different end, but the other influences - Nancarrow's player-piano studies, West African music, jazz - are all fused into a single, indefinable and totally personal language.

The technical challenges are truly formidable. Aimard's performances, prefaced here with crystalline accounts of six of Debussy's Etudes, were astonishing in their ability to see beyond the thickets of notes to the musical shapes, beguiling in their opalescent tone colours and ferocious in their dynamic range. This was constantly remarkable playing.